1. Field
The present disclosure relates to quantum computing and, more particularly, to scalable computing architectures with coupled donor-quantum dot qubits.
2. Description of Related Art
Electron and nuclear spins of donors in silicon have long been recognized as promising qubit candidates. In isotopically purified 28Si they exhibit long coherence times on the order of seconds and their integration can benefit from the great fabrication finesse of silicon nanotechnology. Several concepts for scalable quantum computer architectures with donor spin qubits have emerged. In one proposed aspect of quantum computing (QC), quantum information may be stored in the nuclear spin of phosphorus atoms. Electrostatic gates facilitate transfer of quantum information from nuclear to electron spins and between electron spins, by modulating the contact hyperfine interaction (A-gates), and the Heisenberg exchange coupling (J-gates), respectively. Recently, reliable detection of single electron spins and the control of single electron and nuclear spin states has been observed. For the development of a large scale quantum computer, elements of quantum memory, quantum logic and efficient quantum communication channels may be integrated. While single donor electron and nuclear spin readout and control have been demonstrated, the mastering of spin qubit coupling so that two and multiqubit logic operations can be implemented is still needed. In early concepts of donor qubits, coupling was envisioned along 1D chains of nearest neighbor coupled qubits. While this may suffice for two or three qubit logic demonstrations, severe limitations of nearest neighbor coupling have been noted. Coherent shuttling of electrons between donors has been proposed as a path to circumvent the nearest neighbor coupling challenges or supplement nearest neighbor coupling with a longer range coupling option. For electron shuttling, two important aspects include spin coherence of donor electron and nuclear spins during cycles of ionization and recombination. Other potential paths for long range transport of quantum information from donor spins include concepts of a spin bus, virtual phonon mediated coupling, coupling via nano-mechanical resonators and spin-to-photon coupling in optical cavities or via high Q microwave resonators.
In parallel to single donor spin control, control of electron spins in silicon and Si—Ge based quantum dots has developed. Here, quantum information can be encoded, e. g., in the spin state of a coupled pair of electrons in a double quantum dot structure. For spin based quantum computers with donors, the nuclear spin represents a promising mode for quantum memory. Electrons of donors and dots allow fast single qubit operation and nearest neighbor two-qubit interactions through controlled exchange coupling. Cluster state quantum computer approaches offer an alternative approach e. g. with nuclear spin memory that “only” require nearest neighbor interactions and reliable single qubit control and readout.